A conventional image forming apparatus has a photoreceptor drum for carrying an electrostatic latent image in the main body of the apparatus, and forms an electrostatic latent image by uniformly charging the surface of the photoreceptor drum with a charging device and then exposing the surface in accordance with image information. The electrostatic latent image formed on the surface of the photoreceptor drum is developed into a toner image (developing image) by the developing device, and the toner image is transferred to a transfer material transported according to a timing. Next, the transfer material is separated from the photoreceptor drum by a separating device, and the toner image on the transfer material is fixed by a fixing unit.
In the developing device for use in such a conventional image forming apparatus, a two-component developer composed of toner and carrier is widely used as the developer, and generally the toner is made of colored thermoplastic particles and the carrier is made of ferromagnetic particles such as iron powder.
In the two-component developer composed of toner and carrier, the weight % of toner is several %, and the toner in the developer is consumed every time an electrostatic latent image on the photoreceptor drum is developed. It is therefore necessary to hold the weight percentage of toner (namely, the toner density) in the developer within a predetermined range by supplying toner by an amount corresponding to the consumed amount.
Hence, the image forming apparatus is constructed to detect the toner density and control the toner density to an appropriate value.
FIG. 1 is a block diagram showing the structure of an image forming apparatus. The toner density in the developer stored in a developing device 4 is detected by a magnetic type (permeability measurement type) toner density sensor (ATC sensor) 10, and a CPU (Central Processing Unit) 13 controls the supply of toner from a toner cartridge 8 by driving a toner cartridge drive motor 21 that is toner supplying means, according to the information about the toner density detected by the ATC sensor 10.
FIG. 2 is a graph showing the relationship between the toner density (wt %) and the output voltage value (V) of the ATC sensor 10. When the CPU 13 detects a decrease in the toner density in the developer in the developing unit 4 from the output voltage value of the ATC sensor 10, the CPU 13 drives the toner cartridge drive motor 21 and supplies toner to the developing device 4 through the toner cartridge 8. As a result of this supply, when the toner density increases and the output voltage value of the ATC sensor 10 becomes lower than a reference value (reference output voltage value) V0 shown in FIG. 2, for example, 2.5 V, the CPU 13 stops the toner cartridge drive motor 21 to stop the supply of toner from the toner cartridge 8, and holds the toner density in the developer in the developing device 4 at 4.0 wt % (reference value V0), for example.
However, since the two-component developer has an environmental dependency, in a high humidity state, the toner density in the developer increases and the charge amount decreases, and consequently toner scattering, adhesion of toner in a non-image area, a broken image, etc. occur, and the image quality is lowered. On the other hand, in a low humidity state, the toner density in the developer decreases and the charge amount increases, and consequently the image density decreases and the image quality is lowered due to occurrence of blurred characters, for example.
Moreover with an increase in the copy volume and an increase in the developer agitation time, the developer deteriorates, the toner density increases compared to the initial density (at the time the apparatus was installed and the developer was replaced) due to this deterioration, and the charge amount decreases. Consequently, toner scattering, adhesion of toner to a non-image area, a broken image, etc., occur, and there arises a problem that the image quality is lowered.
In order to solve the above-mentioned problems, a technique is disclosed (in Japanese Patent Application Laid-Open No. 2001-922237) to always hold the toner density in the developer at a uniform value by providing a humidity sensor, correcting a change in the toner density due to a humidity change by changing the reference output voltage value of the ATC sensor 10 according to the humidity change, counting the developer agitation time, and adding a change in the output voltage value of the ATC sensor 10 to a correction value corresponding to the humidity change in accordance with a deteriorated state of the developer caused by the agitation of the developer.
In recent years, with the development of techniques for achieving high image quality, there is a tendency towards a decrease in the particle diameter of toner, and a toner having an average particle diameter of 8 μm or less and a shaper particle diameter distribution has been developed. Accordingly, there is a tendency towards a decrease in the particle diameter of carrier. Further, in order to reduce the toner consumption per copy, it has been proposed to increase the density (content) of pigment in the toner, which was between 5 and 6% conventionally, to 8 to 20% or so.
Thus, with a decrease in the particle diameter of toner and carrier, the specific area of the developer per unit weight becomes larger. Further, with an increase in the pigment density, a change in the behavior of the developer against a change in humidity environment becomes larger accordingly.
In Japanese Patent Application Laid-Open No. 2001-92237, as shown in FIG. 2, when the humidity changes to the higher humidity side, a correction is made to increase the reference value V0 for controlling the toner density to Vb, whereas when the humidity changes to the lower humidity side, a correction is made to decrease the reference value V0 to Vc.
In the image forming apparatus according to Japanese Patent Application Laid-Open No. 2001-92237, when the humidity changes, a toner patch image is formed on a photoreceptor drum under a preset image forming condition as an actual condition for changing the reference output voltage value of the ATC sensor 10, and the CPU 13 corrects a development bias voltage value to hold the image density at a uniform value, based on a density detection result of a photo sensor 12 for detecting the density of the toner patch image, stores the corrected development bias voltage value, and then changes the reference output voltage value of the ATC sensor 10 when the corrected development bias voltage value changes to a predetermined value or larger.
In this toner density correction method, when the state where the difference between a development bias voltage value determined in this image density correction and a development bias voltage value determined in the previous image density correction is not more than a predetermined value occurs repeatedly, the reference output voltage value is not changed even though there is a change in humidity environment, and an appropriate correction of the toner density is not performed.
FIG. 3 is a graph showing the relationship between the output voltage value of the ATC sensor 10 and the developer agitation time. As shown in FIG. 3, even when the toner density is uniform, the output voltage value of the ATC sensor 10 increases with an increase in the agitation time of the developer in the developing device 4. It is considered that this phenomenon occurs due to a phenomenon (spent toner) in which the toner firmly adheres around the carrier. When the toner is supplied by controlling the toner density without considering the increase amount of the output voltage value of the ATC sensor 10, the toner density rises with an increase in the developer agitation time, and the charge amount decreases. Consequently, toner scattering, adhesion of toner to a non-image area, a broken image, etc. occur, and there arises a problem that the image quality is lowered.
In the image forming apparatus according to Japanese Patent Application Laid-Open No. 2001-92237, in accordance with the agitation time of the developer, the reference output voltage value of the ATC sensor 10 is changed by adding a correction value based on a change in the output voltage value of the ATC sensor 10 corresponding to the deteriorated state of the developer of the ATC sensor 10 to a correction value corresponding to a humidity change, and thereby controlling the toner density appropriately. However, this image forming apparatus does not take into account an increase in the output voltage value of the ATC sensor 10 caused by the above-mentioned spent toner. In other words, even when the agitation time increases, if the humidity decreases, control is performed to decrease the reference output voltage value without considering the influence of spent toner, and consequently there arises a problem that the toner density increases.
Moreover, there is a difference in the agitation stress of developer per unit time depending on the frequency of use of the image forming apparatus for copying or printing by the user. The difference in the agitation stress of developer per unit time causes a difference in the charge amount of toner, and consequently causes a difference in the output voltage value of the ATC sensor 10 regardless of the same toner density. If the frequency of use is such that the agitation stress per unit time is low, the toner density in the developer increases and the charge amount of toner decreases, and consequently toner scattering, adhesion of toner to a non-image area, a broken image, etc. occur, and the image quality is lowered. On the other hand, when the frequency of use is such that the agitation stress per unit time is high, the toner density in the developer decreases and the charge amount of toner increases, and consequently the image density decreases and the image quality is lowered due to occurrence of blurred characters, for example.